The present invention relates in general to frequency conversion and, more particularly, to a mixer for converting an input signal having a frequency to an output signal having a different frequency.
Most if not all wireless communication systems transmit and receive information on a modulated carrier frequency. The RF signal transmitted across the air-ways is a high frequency carrier modulated by the signal level of the transmitted information. The transmitter and receiver in the wireless communication system must frequency up-convert baseband information for transmission as an RF signal and frequency down-convert the RF signal back to baseband.
A mixer is typically used to perform the frequency up-conversion and frequency down-conversion. The mixer receives an information input signal and a local oscillator (LO) input signal and produces an output signal as the sum and difference of the frequencies of the input signals. The sum provides frequency up-conversion and the difference provides the frequency down-conversion. A filter is used to select the sum frequency or difference frequency.
One prior art mixer includes a differential transistor pair having gate terminals coupled for receiving the information input signal. A switching circuit is coupled between the drains of the differential transistor pair and a reference current source. The location of the switching circuit increases the power supply potential needed to operate the mixer while avoiding saturation of the switching circuits, current sources, and/or differential transistor pair. Moreover, the location of the switching circuit requires the DC level of the information input signal to be less than the power supply potential in order to avoid saturation of the switching circuits, current sources, and/or differential transistor pair.
Thus, a need exists for a mixer that operates with a reduced power supply potential and allows the DC level of the information input signal to operate at the power supply potential.